Calling and warning apparatus for communication systems



Nov. 22, 1949 H. SELINGER CALLING AND WARNING APPARATUS FORCOMMUNICATION SYSTEMS Filed Jan. 17, 1946 2 Sheets- Sheet l Sill/DING Esmr/om FIG. I

INVENTOR HERBER SELINGER BY ATTQRNEY Nov. 22, 1949 sELlNGER 2,489,202

CALLING AND WARNING APPARATUS FOR COMMUNICATION SYSTEMS Filed Jan. 17,1946 2 Sheets-Sheet 2 Java/1m Sr/V770 FIG. 4

INVENTOR HERBERT SELINGER BY ORNEY Patented Nov. 22, 1949 CALLING ANDWARNING APPARATUS FOR COMMUNICATION SYSTEMS Herbert Selinger, Sydney,New South Wales, Australia, assignor to Hartford National Bank & TrustCompany, Hartford, Conn., trustee Application January 17, 1946, SerialNo. 641,843 In Australia January 19, 1945 13 Claims.

This invention relates to apparatus for producing calling and warningsignals for communication systems in which thermionic vacuum tubeamplifiers are used at the subscriber stations, and particularly tothose systems using a talk-listen switch for the purpose of convertingthe amplifier from a sending condition to a receiving condition and viceversa.

In such systems it is desirable to incorporate means whereby an audibletone may be produced by the receiving equipment when a station is calledand means whereby an audible warning is given when the connectionbetween two stations is broken. The need for an audible warning that theconnection is broken is particularly important in communication systemsin which the subscriber has to perform some action on the completion ofa conversation to restore the equipment to a condition that is suitablefor the reception of another call.

It is the primary object of the invention to provide means wherebycalling and warning tones as above mentioned may be produced.

According to one embodiment of the invention, the warning and callingtones are produced by the generation of audio frequency oscillations inthe receiving amplifier when a calling signal is received or if thecircuit connection with the calling station is interrupted. According toother embodiments of the invention, either the warning tone or thecalling tone is generated by selfoscillation of the receiving amplifierwhilst the other tone is generated by other suitable means.

In performing the invention I contemplate using either one or both oftwo types of circuits for the production of oscillations in thereceiving amplifier.

In one type of circuit, positive feedback is applied from the outputcircuit of the amplifier to the input circuit of the amplifier via ahigh. impedance. The amplifier will generate audio oscillations if theattenuation caused by the said impedance is less than the gain of theamplifier but will not generate oscillations if either the gain of theamplifier is made less than the attenuation or if the attenuation ismade greater than the gain. If the impedance across. the input circuitof the amplifier is of relatively high value oscillations may beproduced but if the impedance across the said input circuit is madesmall in value, c. g. by the connection of a line, the attenuation ofthe feedback voltage may be so great that the generation of oscillationsis prevented.

Aternatively, if the input impedance is high, givand the first tube ofthe amplifier has a large negative bias applied to its control grid, thegain of the amplifier may be insufiicient to generate oscillations.

In the other type of circuit, the positive feedback voltage is appliedto a preceding tube-element via a high impedance in series with arectifier such as a diode tube which may be suitably back biased. Inthis circuit, oscillations will not be generated unless the back biasvoltage of the rectifier is removed or unless a voltage from some othersource is applied across the rectifier and renders it conductive.

In the second type of circuit it is convenient to make use of a tubeknown commercially as the 6G8 in which a diode tube and an amplifyingtube are incorporated in the same envelope, obtaining their electroncurrents from the same cathode. With such a tube, the positive feedbackvoltage may be applied to the cathode element via the anode of thediode, and the diode may be rendered conductive by applying a suittablepositive voltage to its anode via an impedance to enable the circuit togenerate oscillations.

In order that the invention may be more readily understood, referencewill now be made to the accompanying drawings which show the circuitdiagrams of certain applications of the foregoing circuits for theproduction of oscillations toachieve the object of the invention.

In the drawing:

Fig. 1 is a schematic diagram of one preferred embodiment of theinvention,

Fig. 2 is a schematic diagram of a second preferred embodiment,

Fig. 3 is a schematic diagram of a third preferred embodiment, and

r Fig. 4 is a schematic diagram of a fourth preferred embodiment.

In Figure 1 of the drawings the numeral l indicates the first tube andthe numeral 2. the second tube of a resistance coupled amplifier. Apositive feedback network comprising the resistor 4 and the capacitor 3provides an alternating current circuit between the grid of the tube Iand the anode of the tube 2. A negative bias voltage is applied to thegrid of the tube 1 by means of the resistive network comprising theresistors 5 and 9, this bias voltage being sufficiently large to cut oilthe anode cathode current through the tube I.

The grid of the tube I is also connected to the input terminal 1 via theresistor 6 which may be shortcircui'ted by the switch II. In the circuitarrangement shown the other input terminal 8 is connected to earth. Aswitch Ill, linked to the switch II, is also provided and is connectedbetween the junction point of the resistors 5 and 9 and earth.

This circuit arrangement functions in the following manner:

When the switches II and I are open and the direct current resistancebetween the terminals l and 8 is of a relatively high value, the tube Iwill be in a cut-off condition and the amplifier will be silent.

If a component having a relatively low direct current resistance isconnected across theterminals I and 8 such as, for example, the outputwinding of a remote transformer, the bias voltage applied to the grid ofthe tube I will be reduced. If the resistors 6, and 9 are suitablyproportioned and if the negative bias voltage is of a suitable value,the reduction in the effective bias voltage will make the tube Ioperative and the amplifier will oscillate as a result of the feedbackproduced by the feedback network 3, 4. The oscillations so produced canbe used as a calling signal to indicate that a circuit has beenconnected across the terminals 1 and 8.

If the switches II and II) are then closed the bias voltage applied tothe grid of the tube I will be further reduced but the feedback voltagewill be attenuated owing to the reduction in the value of'thealternating current impedance across the terminals I and 8. The resistor4 and the alternating current impedance across the terminals 1 and 8 maybe so proportioned that under these conditions the positive feedbackvoltage is insuflicient to cause oscillation and the amplifier willfunction normally.

When the remote circuit is disconnected from the terminals 1 and 8, thepositive feedback voltage will increase and the amplifier will againoscillate, thus giving a warning that the remote circuit has beendisconnected.

If the switches I0 and II are then opened oscillations will cease andthe amplifier will be restored to its initial state in readiness for afurther call.

The circuit shown in Figure 2 is a modification of that of Figure 1, thearrangement being one which provides for a calling tone and a warningtone, thereby permitting the use of two different tones for calling thewarning purposes. This circuit differs from that of Figure 1 in that theresistor 6 and the switch I! are omitted and the junction point ofresistors 5 and 9 constituting a resistive network is connected througha blocking capacitor I I to an auxiliary source of oscillations I2.

With this circuit the connection of a remote circuit of low alternatingcurrent impedance across the terminals 1 and 8 renders the tube Ioperative but does not permit self oscillation as the positive feedbackvoltage is attenuated owing to the impedance of the remote circuit. Onthe other hand, the auxiliary oscillations from the source I2 areapplied to the grid of the tube I and so provide a calling signal. Byclosing the switch II] the calling tone is eliminated and the amplifiermay be used in a normal manner. The removal of the remote circuit fromacross the terminals 7 and 8 increases the feedback voltage and causesthe amplifier to oscillate thereby to produce a warning signal. Byopening the switch I0 the warning signal is cut out and the originalconditions are restored.

With this arrangement, the tone of the calling signal is determined bythe frequency of the 4 auxiliary oscillator while the tone of thewarning signal is dependent upon the characteristics of the amplifier.

Figure 3 depicts another embodiment of the invention in which use ismade of a type 6G8 tube. In this arrangement the numeral I representsthe 6G8 tube and the numeral 2 the second tube of the amplifier. Theoutput transformer of the amplifier has a secondary winding 4 which isconnected to the diode anode II, of the tube I by the blocking condenser3. The diode anode is connected to a source of positive voltage by theresistors 6 and IS. A resistor 9 is connected from the junction point ofresistors 6 and I6 to earth Via a switch II).

The input terminals are indicated by the numerals 1 and 8, the latterterminal being connected to earth and the terminal I being connected tothe grid of the. tube I. The grid of the tube I is also connected to asuitable source of negative voltage via the resistive network comprisingthe resistors 5 and I2, a resistor I4 in series with a switch I3 beingconnected between the junction point of resistors 5 and I2 and earth.The switch I3 is linked to the switch I0.

When open circuit conditions exist between the terminals 1 and 8 thebias voltage applied to the grid of the tube I is sufficient to renderthe tube inoperative.

When a remote circuit is connected across the terminals 1 and 8, thetube I becomes operative as a result of the reduction in the grid biasvoltage. With the switch Ill in its open position, positive feedback isapplied from the winding 4 via the blocking condenser 3 and the diodeanode II, and if the diode anode is positive with respect to thecathode, the amplifier will oscillate and produce a calling signal.

If the switch It is closed the positive voltage applied to the diodeanode II will be reduced and, if the components have suitable values,the amplifier will function in normal manner.

If the remote circuit connected across the 7 terminals 1 and 8 isremoved, the current through the tube I will decrease because thenegative bias will increase to a value determined by resistors I2 andI4. Consequently, the Voltage at the cathode of the tube I, which ismaintained positive with respect to earth by the cathode resistor I5,will decrease to a value which can be less than the voltage at the diodeanode II if the components have suitable values. The diode will thusbecome conductive and cause the circuit to oscillate thereby producing awarning signal. By opening the switches I0 and I3 the circuit isrestored to its initial condition.

The circuit in Figure 4 is that of an embodiment of the invention inwhich a calling signal is produced. In this circuit positive feedback isapplied to the screen of the first tube of the amplifier from thesecondary winding 2 of the output transformer via the switch I5, theresistor 4, and the capacitor 3. A bias voltage suilicient to bias thefirst tube beyond cut-off is applied to the grid of the tube Via theresistor 5. When a remote circuit is connected across the terminals 1and 8 the bias voltage is removed from the first tube and the amplifierwill oscillate thereby to produce a calling signal. When the signal isanswered, the switch I5 is opened, thus breaking the positive feedbackcircuit and permitting the amplifier to function normally.

If desired, time delay filters may be employed in the circuits abovedescribed in order to prevent the generation of calling or warningsignals as a result of momentary actions such as when a switch is openedor closed.

In the circuit diagrams illustrated, those components well known in theart but not forming portion of the invention have been omitted for thesake of clarity.

Direct current voltages are shown as though produced by batteries butthey may be produced by resistor networks in any suitable manner.

It is not intended to limit the invention to circuit arrangements inwhich control voltages are applied to the control grid electrodes as itwill be obvious to those skilled in the design of amplifiers thatsimilar effects may be obtained by means of Voltages applied to otherelectrodes.

Having now described my invention what I claim as new and desire tosecure by Letters Patent is:

1. Calling and warning apparatus for a communication system including aVacuum tube amplifier at a receiving station and a sending stationarranged for connection to the input circuit of said amplifier,comprising means for rendering the amplifier inoperative until aconnection is made with the sending station, and for rendering theamplifier operative by the connection made at the sending station;tone-producing means at said receiving station; means whereby audiofrequency oscillations may be generated, means for introducing saidoscillations into said amplifier so that a calling tone may be given bysaid tone-producing means when a call from said sending station isreceived, and means whereby the generation of the oscillations may bediscontinued so as to permit the amplifier to function in a normalmanner.

2. Apparatus according to claim 1, wherein said amplifier includes avacuum tube having a cathode, a grid and an anode, and the means forrendering the amplifier inoperative comprises a resistive networkconnected to apply a voltage to said grid so that the anode cathodecurrent through the tube is substantially zero.

3. Apparatus according to claim 1, wherein said amplifier includes avacuum tube having a cathode, a grid and an anode, and the means forrendering the amplifier inoperative comprises a resistive networkconnected to apply a voltage to said grid so that the anode cathodecurrent through the tube is substantially zero, said resistive networkbeing connected in parallel with the input terminals of the amplifier sothat, when a connection is made with the sending station, the constantsof the network are altered to vary the voltage applied to said grid toan extent permitting the anode cathode current through the tube toincrease to render the amplifier operative.

4. Apparatus according to claim 1, wherein the means whereby the audiofrequency oscillations may be generated includes a feed back networkconnected to inject a feed back voltage into the amplifier thereby tocause the amplifier to generate the oscillations.

5. Calling and warning apparatus for a communication system including avacuum tube amplifier at a receiving station, said amplifier beingprovided with a vacuum tube having a cathode, a grid and an anode, and asending station arranged for connection to the input circuit of saidamplifier, comprising means for rendering the amplifier inoperativeuntil a connection is made with the sending station, and for renderingthe amplifier operative by the connection made at the sending station;tone-producing means at said receiving station; a feedback networkconnected to inject a feedback voltage into the amplifier thereby tocause the amplifier to generate low frequency oscillations so that acalling tone may be given when a connection is made with said sendingstation, said means comprising a resistive network connected to apply avoltage to said grid so that the anode cathode current through the tubeis substantially zero, the resistive network being connected in parallelwith the input terminals of the amplifier so that, when a connection ismade With a sending station the constants of the network are altered tovary the voltage applied to said grid to an extent permitting the anodecathode current through the tube to increase to render the amplifieroperative, and switch means whereby the constants of the circuits inparallel with the input terminals may be changed thereby to attenuatethe feedback voltage so that the generation. of the oscillations may bediscontinued to permit the amplifier to function in a normal manner.

6. Apparatus according to claim 5, wherein the constants of th resistivenetwork having values at which, when the connection with the sendingstation is broken, the generation of the oscillations recommences sothat a warning tone may be given.

7. Apparatus according to claim 1 and including an auxiliary source togenerate the low frequency oscillations, in' combination with switchmeans whereby the oscillations may be eliminated.

8. Apparatus according to claim 1, and including an auxiliary source togenerate the low frequency oscillations, in combination with switchmeans whereby the oscillations may be eliminated, and a feedback networkconnected. to inject a feedback voltage into the amplifier thereby tocause the amplifier to generate low frequency oscillations so as to givea warning tone when the connection with the sending station is broken.

9. Calling and warning apparatus for a communication system including avacuum tube amplifier at a receiving station, and a sending stationarranged for connection to the input circuit of said amplifier,comprising means for rendering the amplifier inoperative until aconnection is made with the sending station, and for rendering theamplifier operative by the connection made at the sending station;tone-producing means at said receiving station; a feedback networkconnected to inject a feedback voltage into the amplifier via arectifier of the diode type thereby to cause the amplifier to generateaudio frequency oscillations so that a calling tone may be given when acall is received, and means whereby the generation of the oscillationsmay be discontinued so as to permit the amplifier to function in anormal manner.

10. Apparatus according to claim 9, wherein the feedback network iscombined with switch means whereby the voltage applied to the diodeanode may be reduced so that the generation of the oscillations may bediscontinued.

11. Calling and warning apparatus for a communication system including avacuum tube amplifier, comprising means for rendering the amplifierinoperative until a connection is made with a sending station, and forrendering the amplifier operative by the connection made at the sendingstation; tone-producing means at said receiving station; a feedbacknetwork which is connected to inject a feedback voltage into theamplifier via a rectifier of the diode type thereby to cause theamplifier to generate audio frequency oscillations so that a callingtone may be given when a call is received, said means comprising aresistive network which is connected to apply a Voltage to at least oneof the electrodes of a vacuum tube in the amplifier so that the anodecathode current through the tube is substantially zero, the constants ofthe resistive network having values at which when the connection withthe sending station is broken, the generation of oscillationsrecommences so that a warning tone may be given.

12. Calling and Warning apparatus for a communication system including avacuum tube amplifier at a receiving station, and a sending stationarranged for connection to the input circuit of said amplifier,comprising means for rendering the amplifier inoperative until aconnection is made with the sending station and for rendering theamplifier operative by the connection made at the sending station, toneproducing means at said receiving station, and for rendering theamplifier operative by the connection made at the sending station;tone-producing means at said receivin station; and a feedback networkcoupled between the output and input circuits of said amplifier toinject a feedbag voltage into the amplifier thereby to cause theamplifier to generate audio frequency oscillations exciting said callingtone means when a call is received, said feedback network includingswitch means whereby the path for the feedback voltage may be broken sothat the generation of the oscillations may be discontinued to permitthe amplifier to function in a normal manner.

13. Calling and warning apparatus for a com- 8 munication system,comprising a vacuum tube amplifier at a receiving station; means forrendering the amplifier inoperative until a connection is made at acalling station arranged for coupling to the input circuit of saidamplifier to complete the circuit therebetween; means operative inresponse to such connection for locally generating at the receivingstation an audiofrequency tone to indicate a connection has been made tothe receiving station; means at the receiving station for thenterminating such connection-indicating tone and for rendering theamplifier operative for normal functioning; and means including saidconnection-tone-terminating means and responsive to disconnection of thecalling station for establishing a warning tone on said tone-producingmeans to indicate such disconnection and to maintain said warning toneuntil a predetermined operation of the toneterminating means isperformed.

HERBERT SELINGER.

REFERENCES CITED The following references are of record in the file ofthis patent:

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